1. Field of the Invention
The present invention relates to an image formation apparatus which performs image formation by using a light emission element array such as an LED array.
2. Related Background Art
Conventionally, a self-scanning type LED (hereinafter called xe2x80x9cSLEDxe2x80x9d) array was introduced by Japanese Patent Applications (Laid-Open) Nos. 1-238962, 2-208067, 2-212170, 3-20457, 3-194978, 4-5872, 4-23367, 4-296579, and 5-84971, Japan Hard Copy ""91 xe2x80x9cLight Emission Element Array for Optical Printer in Which Driving Circuits are Integratedxe2x80x9d, Proceedings of the 1990 IEICE (The Institute of Electronics, Information and Communication Engineers) Spring Conference (Mar. 5, 1990), xe2x80x9cSelf-Scanning Type Light Emission Element (SLED) Using PNPN Thyristor Structurexe2x80x9d, and the like. Namely, the SLED has been remarked recently as a light emission element for used in recording.
Further, in such an image formation apparatus which is applied as a recording head disposed like the above LED array, if gradation is given to a written pixel, it is necessary to independently modulate a light emission current (i.e., a driving current) of each light emission element on the basis of multivalue image data, or it is necessary to independently modulate the length of a period L or a duty ratio of a signal "PHgr"I on the basis of the multivalue image data. To perform such modulation, it is necessary to provide light emission current modulation means or light emission duty modulation means the number of which corresponds to the number of the light emission elements which simultaneously emit light on one head. Therefore, if such means is provided, the driving circuit becomes complicated and large in size.
Further, in the above-mentioned SLED, one light emission enable signal is input to one LED array chip. Therefore, it is necessary to provide plural modulation means the number of which corresponds to the number of LED array chips to be installed.
Further, in such an SLED head as above, a scan is independently performed by light emission of each LED array chip. Thus, according as a photosensitive drum is rotated in a sub-scan direction, a writing line on the photosensitive drum by the LED array head becomes inclined for each LED array chip, whereby one scan line (main-scan line) becomes like sawteeth.
This problem will be explained with reference to FIGS. 21 and 22.
FIG. 21 is an explanatory view which schematically shows a photosensitive body and an optical writing unit of an electrophotographic-system image formation apparatus which uses the LED array head as an optical writing means.
A cylindrical photosensitive drum 342 rotates at predetermined constant speed (i.e., a rotating speed Vd) in the direction indicated by the arrow when an image is formed. Numeral 210 denotes an LED array head which is disposed opposite the photosensitive drum 342 and on which plural LED array chips 211 are disposed. The direction along which the LED array chips 211 are disposed is substantially the axial direction of the photosensitive drum 342. Plural (i.e., 128 dots in this case) light emission pixels are aligned parallel to the chips along the longitudinal direction of the chip. The LED array chips 211 are aligned and disposed on a base substrate (not shown), and necessary driving signals are supplied to the chips 211 through not-shown bonding wires. In this explanation, the plural (i.e., 12 for simplification) LED array chips 211 are aligned on the base substrate.
Each LED array chip 211 scans the light emission points in the direction indicated by the arrow. Namely, the light emission pixels are sequentially shifted. Thus, according to the light emission of the LED array chips 211, optical writing (i.e., exposure) is performed on the photosensitive face of the opposite photosensitive drum 342. Such a main scan is repeated according to the rotation of the drum 342, whereby a sub-scan is performed.
FIG. 22 is an explanatory view which shows an example of the optical writing line formed on the photosensitive drum 342, together with the LED array chips 211.
The rotating speed of the photosensitive drum is assumed to be Vd, a repetition period in the sub-scan is assumed to be Ts, and resolution (light emission pixel pitch) in the main scan is assumed to be Pm. Generally, the repetition period in the sub-scan is set according to the following relation, and the resolution (i.e., the repetition period) in the sub-scan is suited to the resolution of the main scan:
Pm=Vdxc3x97Ts
However, actually the scan line by the respective chips becomes a sawtooth-like line as shown in FIG. 22 including a nonconformity (or gap) represented by Vdxc3x97Ts (=Pm) in the sub-scan direction. This nonconformity (or gap) corresponds to one pixel pitch of the main-scan light emission pixel. If there are such sawteeth in the line, for example, when guidelines and characters in the main scan direction are printed, an unnatural nonconformity or gap occasionally appears on the periphery. Further, if a screen-processed gray image is written, a luminous beat occasionally appears in the screen image frequency at the nonconformity or gap portion.
Of course, the above problem due to the sawtoothed nonconformity is not specific to image formation apparatus using the SLED head. Namely, the same problem occurs in the LED array head structured to independently move and scan the light emission point in each of the plural divided area of the head and to perform optical writing.
An object of the present invention is to solve the above-mentioned conventional problems.
In order to achieve the above object, the present invention provides an image formation apparatus which records an image on a recording medium, comprising a recording element array head on which plural recording chips are disposed, in the recording chip plural recording elements being arrayed, main scan means, and sub-scan means. The main scan means scans the recording elements n (nxe2x89xa72) times on the basis of image data corresponding to one scan, and the direction along which the recording elements are disposed is deviated from the direction perpendicular to the sub-scan direction, by a predetermined angle.
Other objects and features of the present invention will become apparent from the following detailed description and the attached drawings.